What Causes The Northern Lights

Solar flares don't cause auroras. Traveling at the speed of light, radiation from a solar flare takes minutes to reach Earth. The northern lights are caused by the solar wind, a constant stream of charged particles flowing from the sun that interacts with Earth’s magnetic field. While most are deflected, some particles spiral along magnetic field lines toward the poles, colliding with oxygen and nitrogen atoms high in the atmosphere. These collisions excite the gases, causing them to release energy as shimmering light.

The root cause of geomagnetic disturbances is CMEs, which often follow a solar flare. During a CME, a cloud of super-charged particles leaves the solar surface and travels across the solar system, though at a much slower speed than the radiation from a solar flare.

Forecasting The Northern Lights

The aurora borealis is notoriously difficult to predict. That's because it's very difficult to know whether a CME is Earth-bound and, if it is, exactly when it will arrive. After all, there are no satellites close to the sun to detect the speed of a CME. That data can only be collected when the CME's effect on the solar wind — the stream of charged particles released from the sun that travels in all directions in the solar system — is measured by NOAA’s DSCOVR satellite, which orbits Earth. DSCOVR measures the solar wind’s speed and magnetic intensity, which is critical in calculating how it is about to change. Only then can an aurora display be accurately forecast by NOAA's Space Weather Prediction Center, but there's not much warning — just 30 minutes.

A Waning ‘Solar Maximum’

The sun has a roughly 11-year cycle in which its magnetic activity peaks and troughs. How active it is is measured by counting sunspots on its surface, dark patches that indicate complexity — and often produce solar flares and CMEs. NASA and NOAA’s Solar Cycle 25 Prediction Panel announced the arrival of solar maximum in October 2024, though the exact date won't be obvious for a few years.

However, the declining phase of the solar maximum can see extreme solar events. Even when the number of sunspots begins to decrease, big displays of aurora are possible.

Solar Cycle 25’s Most Powerful Solar Flares

Although an X2.4-class solar flare is the biggest for months, there have been much stronger events in the current solar cycle:

• Oct. 3, 2024 — X9 (the strongest of solar cycle 25, which caused global auroras).
• May 5, 2024 — X8.7
• Oct. 1, 2024 — X7.1
• Feb. 22, 2024 — X6.3
• May. 11, 2024 — X5.9 (caused global auroras).

Solar flares are graded according to their severity, with B-class, followed by C, M and X, according to NASA. Each letter represents a tenfold increase in energy output. The strongest ever solar flare detected was an estimated X28 on Nov. 4, 2003.

Northern Lights Updates

Aurora-chasers frequently use the Kp index to predict the intensity of a geomagnetic storm, but for aurora displays, the interplanetary magnetic field’s Bz component is more important. Bz determines how easily solar energy enters Earth’s magnetosphere. When Bz points north, Earth’s field resists it; when Bz swings south, the two fields connect, allowing plasma to stream in. A sustained southward Bz of −5 nT or stronger usually signals an imminent display of aurora.

To check visibility in real time, use NOAA’s 30-minute aurora forecast or download apps such as Aurora Now, My Aurora Forecast or Glendale Aurora for up-to-the-minute alerts and live solar wind data.

Wishing you clear skies and wide eyes.